Methods and apparatus for treating fibrillation and creating defibrillation waveforms

ABSTRACT

Methods and apparatus for treating fibrillation utilize biphasic waveforms. A cardiac stimulator includes a defibrillation circuit that uses a pulse width modulated capacitive discharge to generate various biphasic waveforms, one or more of which may be delivered to the heart to treat the fibrillation.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.09/371,279, filed Aug. 10, 1999, now issued as U.S. Pat. No. 6,298,266.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to cardiac stimulators and, moreparticularly, to cardiac stimulators having the ability to treatfibrillations.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects ofart which may be related to various aspects of the present inventionwhich are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

As most people are aware, the human heart is an organ having fourchambers. A septum divides the heart in half, with each half having twochambers. The upper chambers are referred to as the left and rightatria, and the lower chambers are referred to as the left and rightventricles. Deoxygenated blood enters the right atrium through theinferior and superior vena cava. Contraction of the right atrium and ofthe right ventricle pump the deoxygenated blood through the pulmonaryarteries to the lungs where the blood is oxygenated. This oxygenatedblood is carried to the left atrium by the pulmonary veins. From thiscavity, the oxygenated blood passes to the left ventricle and is pumpedto a large artery, the aorta, which delivers the pure blood to the otherportions of the body through the various branches of the vascularsystem.

In the normal human heart, the sinus node (generally located near thejunction of the superior vena cava and the right atrium) constitutes theprimary natural pacemaker by which rhythmic electrical excitation isdeveloped. The cardiac impulse arising from the sinus node istransmitted to the two atrial chambers. In response to this excitation,the atria contract, pumping blood from those chambers into therespective ventricles. The impulse is transmitted to the ventriclesthrough the atrioventricular (AV) node to cause the ventricles tocontract. This action is repeated in a rhythmic cardiac cycle in whichthe atrial and ventricular chambers alternately contract and pump, thenrelax and fill. One-way valves between the atrial and ventricularchambers in the right and left sides of the heart and at the exits ofthe right and left ventricles prevent backflow of the blood as it movesthrough the heart and the circulatory system.

The sinus node is spontaneously rhythmic, and the cardiac rhythmoriginating from the sinus node is referred to as sinus rhythm. Thiscapacity to produce spontaneous cardiac impulses is called rhythmicity.Some other cardiac tissues also possess this electrophysiologic propertyand, hence, constitute secondary natural pacemakers. However, the sinusnode is the primary pacemaker because it has the fastest spontaneousrate and because the secondary pacemakers tend to be inhibited by themore rapid rate at which impulses are generated by the sinus node.

The resting rates at which sinus rhythm occurs in normal people differfrom age group to age group, generally ranging between 110 and 150 beatsper minute (“bpm”) at birth, and gradually slowing in childhood to therange between 65 and 85 bpm usually found in adults. The resting sinusrate, typically referred to simply as the “sinus rate,” varies from oneperson to another and, despite the aforementioned usual adult range, isgenerally considered to lie anywhere between 60 and 100 bpm (the “sinusrate range”) for the adult population.

A number of factors may affect the sinus rate, and some of those factorsmay slow or accelerate the rate sufficiently to take it outside of thesinus rate range. Slow rates (below 60 bpm) are referred to as sinusbradycardia, and high rates (above 150 bpm) are referred to as sinustachycardia. In particular, sinus tachycardia observed in healthy peoplearises from various factors which may include physical or emotionalstress, such as exercise or excitement, consumption of beveragescontaining alcohol or caffeine, cigarette smoking, and the ingestion ofcertain drugs. The sinus tachycardia rate usually ranges between 101 and160 bpm in adults, but has been observed at rates up to (and ininfrequent instances, exceeding) 200 bpm in younger persons duringstrenuous exercise.

Sinus tachycardia is sometimes categorized as a cardiac arrhythmia,since it is a variation from the normal sinus rate range. Arrhythmiarates which exceed the upper end of the sinus rate range are termedtachyarrhythmias. Healthy people usually experience a gradual return totheir normal sinus rate after the removal of the factors giving rise tosinus tachycardia. However, people suffering from disease may experienceabnormal arrhythmias that may require special, and in some instancesimmediate, treatment. In this text, we typically refer to abnormallyhigh rates that have not yet been determined to be caused by myocardialmalfunction as tachycardias and to abnormally high rates that have beendetermined to be caused by myocardial malfunction as tachyarrhythmias.

It should also be appreciated that an abnormal tachyarrhythmia mayinitiate fibrillation. Fibrillation is a tachyarrhythmia characterizedby the commencement of completely uncoordinated random contractions bysections of conductive cardiac tissue of the affected chamber, quicklyresulting in a complete loss of synchronous contraction of the overallmass of tissue and a consequent loss of the blood-pumping capability ofthat chamber.

In addition to rhythmicity, other electrophysiologic properties of theheart include excitability and conductivity. Excitability, which is theproperty of cardiac tissue to respond to a stimulus, varies with thedifferent periods of the cardiac cycle. As one example, the cardiactissue is not able to respond to a stimulus during the absoluterefractory phase of the refractory period, which is approximately theinterval of contraction from the start of the QRS complex to thecommencement of the T wave of the electrocardiogram. As another example,the cardiac tissue exhibits a lower than usual response during anotherportion of the refractory period constituting the initial part of therelative refractory phase, which is coincident with the T wave. Also,the excitability of the various portions of the cardiac tissue differsaccording to the degree of refractoriness of the tissue.

Similarly, the different portions of the heart vary significantly inconductivity, which is a related electrophysiologic property of cardiactissue that determines the speed with which cardiac impulse s aretransmitted. For example, ventricular tissue and atrial tissue are moreconductive than AV junction tissue. The longer refractory phase andslower conductivity of the AV junction tissue give it a significantnatural protective function, as described in more detail later.

For a variety of reasons, a person's heart may not function properlyand, thus, endanger the person's well-being. Most typically, heartdisease affects the rhythmicity of the organ, but it may also affect theexcitability and/or conductivity of the cardiac tissue as well. As mostpeople are aware, medical devices have been developed to facilitateheart function in such situations.

For instance, if a person's heart does not beat properly, a cardiacstimulator may be used to provide relief. A cardiac stimulator is amedical device that delivers electrical stimulation to a patient'sheart. A cardiac stimulator generally includes a pulse generator forcreating electrical stimulation pulses and a conductive lead fordelivering these electrical stimulation pulses to the designated portionof the heart. As described in more detail below, cardiac stimulatorsgenerally supply electrical pulses to the heart to keep the heartbeating at a desired rate, although they may supply a relatively largerelectrical pulse to the heart to help the heart recover fromfibrillation.

Early pacemakers were devised to treat bradycardia. These pacemakers didnot monitor the condition of the heart. Rather, early pacemakers simplyprovided stimulation pulses at a fixed rate and, thus, kept the heartbeating at that fixed rate. However, it was found that pacemakers ofthis type used an inordinate amount of energy due to the constant pulseproduction. Even the sinus node of a heart in need of a pacemaker oftenprovides suitable rhythmic stimulation occasionally. Accordingly, if aheart, even for a short period, is able to beat on its own, providing anelectrical stimulation pulse using a pacemaker wastes the pacemaker'senergy.

To address this problem, pacemakers were subsequently designed tomonitor the heart and to provide stimulation pulses only when necessary.These pacemakers were referred to as “demand” pacemakers because theyprovided stimulation only when the heart demanded stimulation. If ademand pacemaker detected a natural heartbeat within a prescribed periodof time, typically referred to as the “escape interval”, the pacemakerprovided no stimulation pulse. Because monitoring uses much less powerthan generating stimulation pulses, the demand pacemakers took a largestep toward conserving the limited energy contained in the pacemaker'sbattery.

Clearly, the evolution of the pacemaker did not cease with the advent ofmonitoring capability. Indeed, the complexity of pacemakers hascontinued to increase in order to address the physiological needs ofpatients as well as the efficiency, longevity, and reliability of thepacemaker. For instance, even the early demand pacemakers providedstimulation pulses, when needed, at a fixed rate, such as 72 pulses perminute. To provide a more physiological response, pacemakers having aprogrammably selectable rate were developed. So long as the heart wasbeating above this programmably selected rate, the pacemaker did notprovide any stimulation pulses. However, if the heart rate fell belowthis programmably selected rate, the pacemaker sensed the condition andprovided stimulation pulses as appropriate.

To provide even further physiological accuracy, pacemakers have now beendeveloped that automatically change the rate at which the pacemakerprovides stimulation pulses. These pacemakers are commonly referred toas “rate-responsive” pacemakers. Rate-responsive pacemakers sense aphysiological parameter of the patient and alter the rate at which thestimulation pulses are provided to the heart. Typically, this monitoredphysiological parameter relates to the changing physiological needs ofthe patient. For instance, when a person is at rest, the person's heartneed only beat relatively slowly to accommodate the person'sphysiological needs. Conversely, when a person is exercising, theperson's heart tends to beat rather quickly to accommodate the person'sheightened physiological needs.

Unfortunately, the heart of a person in need of a pacemaker may not beable to beat faster on its own. Prior to the development ofrate-responsive pacemakers, patients were typically advised to avoidundue exercise, and pacemaker patients that engaged in exercise tendedto tire quickly. Rate-responsive pacemakers help relieve this constraintby sensing one or more physiological parameters of a patient thatindicates whether the heart should be beating slower or faster. If thepacemaker determines that the heart should be beating faster, thepacemaker adjusts its base rate upward to provide a faster pacing rateif the patient's heart is unable to beat faster on its own. Similarly,if the pacemaker determines that the patient's heart should be beatingmore slowly, the pacemaker adjusts its base rate downward to conserveenergy and to conform the patient's heartbeat with the patient's lessactive state.

As noted above, pacemakers have historically been employed primarily forthe treatment of heart rates which are unusually slow, referred to asbradyarrhythmias. However, over the past several years cardiac pacinghas found significantly increasing usage in the management of heartrates which are unusually fast, referred to as tachyarrhythmias.Anti-tachyarrhythmia pacemakers take advantage of the previouslymentioned inhibitory mechanism that acts on the secondary naturalpacemakers to prevent their spontaneous rhythmicity, sometimes termed“postdrive inhibition” or “overdrive inhibition”. In essence, the heartmay be stimulated with a faster than normal pacing rate (1) to suppresspremature atrial or ventricular contractions that might otherwiseinitiate ventricular tachycardia, flutter (a tachyarrhythmia exceeding250 bpm), or fibrillation or (2) to terminate an existingtachyarrhythmia.

Typically, these pulses need only be of sufficient magnitude tostimulate the excitable myocardial tissue in the immediate vicinity ofthe pacing electrode. However, another technique for terminatingtachyarrhythmias, referred to as cardioversion, utilizes apparatus toshock the heart synchronized to the tachyarrhythmia with one or morecurrent or voltage pulses of considerably higher energy content thanthat of the pacing pulses. Defibrillation, a related technique, alsoinvolves applying one or more high energy “countershocks” to the heartin an effort to overwhelm the chaotic contractions of individual tissuesections to allow reestablishment of an organized spreading of actionpotential from cell to cell of the myocardium and, thus, restore thesynchronized contraction of the mass of tissue.

In the great majority of cases, atrial fibrillation is hemodynamicallytolerated and not life-threatening because the atria provide only arelatively small portion (typically on the order of 15 to 20 percent) ofthe total volume of blood pumped by the heart per unit time, typicallyreferred to as cardiac output. During atrial fibrillation, the atrialtissue remains healthy because it is continuing to receive a freshsupply of oxygenated blood as a result of the continued pumping actionof the ventricles. Atrial tachyarrhythmia may also be hemodynamicallytolerated because of the natural protective property of the AVjunctional tissue attributable to its longer refractory period andslower conductivity than atrial tissue. This property renders the AVjunctional tissue unable to respond fully to the more rapid atrialcontractions. As a result, the ventricle may miss every other, orperhaps two of every three, contractions in the high rate atrialsequence, resulting in 2:1 or 3:1 A-V conduction and, thus, maintainrelatively strong cardiac output and an almost normal rhythm.

Nevertheless, in cases where the patient is symptomatic or at high riskin events of atrial tachyarrhythmia or fibrillation, special treatmentof these atrial disorders may be appropriate. Such circumstances mayinclude, for example, instances where the patient suffers fromventricular heart disease and cannot easily withstand even the smallconsequent reduction of ventricular pumping capability, as well asinstances where the rapid atrial rhythm is responsible for anexcessively rapid ventricular rate. The methods of treatment commonlyprescribed by physicians for treating atrial tachyarrhythmia andfibrillation include medication, catheter ablation, pacing therapy,cardiac shock therapy, and in some cases, surgically creating an A-Vblock and implanting a ventricular pacemaker.

In contrast to the atrial arrhythmias discussed above, cardiac outputmay be considerably diminished during an episode of ventriculartachyarrhythmia because the main pumping chambers of the heart, theventricles, are only partially filled between the rapid contractions ofthose chambers. As in the case atrial fibrillation, ventricularfibrillation is characterized by rapid, chaotic electrical andmechanical activity of the excitable myocardial tissue. However, incontrast to atrial fibrillation, ventricular fibrillation manifests aninstantaneous cessation of cardiac output as the result of theineffectual quivering of the ventricles—a condition that typicallyrequires almost immediate treatment.

The type and shape of the defibrillation waveform, as well as itsintensity, determine the efficacy of the waveform in treatingfibrillation. For example, in older defibrillators, such as externaldevices used in emergency situations, a monophasic waveform was used. Atypical monophasic waveform rises from zero volts to some prescribedpositive voltage appropriate to defibrillate the heart. While such awaveform typically overcomes the fibrillation of the heart, if it is notof sufficient intensity refibrillation may occur.

To address this concern, most defibrillators now use a biphasicwaveform. A typical biphasic waveform rises from zero volts to someprescribed positive voltage, and then switches rapidly to someprescribed negative voltage before returning to zero. Biphasic waveformsexhibit several advantages over monophasic wavefonns. For example,because part of a biphasic waveform is at a positive voltage level andpart is at a negative voltage level, a biphasic waveform tends todeliver a more balanced charge than a monophasic waveform. Because amore balanced charge leaves less net charge on the interface between theheart and the electrode, there is less polarization at this boundary.This is a desirable result because the polarization potential of apolarized boundary must first be overcome to deliver the requiredstimulation to the heart, thus increasing the required intensity of thewaveform and the power drain on the cardiac stimulator. Therefore,biphasic waveforms typically require less energy to defibrillate thanmonophasic waveforms.

While biphasic waveforms appear to exhibit greater efficacy thanmonophasic waveforms, various problems still exist. For instance,theoretically speaking, biphasic waveforms may take virtually aninfinite number of shapes. While a variety of biphasic waveforms havebeen considered for defibrillation, no known waveform appears to be bestsuited for every situation. Furthermore, many waveforms remaintheoretical, because no circuit suitable for use in an implantable ICDhas been designed to create the waveform.

The present invention may address one or more of the problems set forthabove.

SUMMARY OF THE INVENTION

Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms the invention might take and that these aspects are notintended to limit the scope of the invention. Indeed, the invention mayencompass a variety of aspects that may not be set forth below.

In accordance with one aspect of the present invention, there isprovided a biphasic defibrillation waveform which includes a positivevoltage phase beginning at about zero volts and having an initialpositive voltage magnitude greater than zero volts. The positive voltagephase has a first positively sloped portion extending from the initialpositive voltage magnitude to a maximum positive voltage magnitudegreater than the initial positive voltage magnitude. A negative voltagephase has an initial maximum negative voltage magnitude less than zerovolts extending from the maximum positive voltage magnitude of thepositive voltage phase. The negative voltage phase has a secondpositively sloped portion extending from the initial maximum negativevoltage magnitude to a terminal negative voltage magnitude greater thanthe initial maximum negative voltage magnitude.

In accordance with another aspect of the present invention, there isprovided a biphasic defibrillation waveform which includes a positivevoltage phase having an initial voltage magnitude equal to about zerovolts and having a first positively sloped portion extending from theinitial voltage magnitude to a maximum positive voltage magnitudegreater than the initial voltage magnitude. A negative voltage phase hasan initial negative voltage magnitude less than or equal to zero voltsextending from the maximum positive voltage magnitude of the positivevoltage phase. The negative voltage phase has a second sloped portionextending from the initial negative voltage magnitude to a terminalnegative voltage having a magnitude less than or equal to zero volts.

In accordance with still another aspect of the present invention, thereis provided a biphasic defibrillation waveform which includes a positivevoltage phase having an initial maximum positive voltage magnitudegreater than zero volts and having a first negatively sloped portionextending from the initial maximum positive voltage magnitude to aterminal positive voltage magnitude less than the initial maximumpositive voltage magnitude. A negative voltage phase has an initialnegative voltage magnitude less than or equal to zero volts extendingfrom the terminal positive voltage magnitude of the positive voltagephase. The negative voltage phase has a second sloped portion extendingfrom the initial negative voltage magnitude to a terminal negativevoltage having a magnitude less than or equal to zero volts.

In accordance with yet another aspect of the present invention, there isprovided a biphasic defibrillation waveform which includes a positivevoltage phase having an initial positive voltage having a magnitudegreater than or equal to zero volts and having a first sloped portionextending from the initial positive voltage to a terminal positivevoltage having magnitude greater than or equal to zero volts. A negativevoltage phase has an initial negative voltage having a magnitude lessthan or equal to zero volts extending from the terminal positive voltageof the positive voltage phase. The negative voltage phase has a secondsloped portion extending from the initial negative voltage to a terminalnegative voltage having a magnitude less than or equal to zero volts.

In accordance with a further aspect of the present invention, there isprovided a method of generating a biphasic defibrillation waveform thatincludes the acts of: generating a positive voltage phase having aninitial positive voltage having a magnitude greater than or equal tozero volts and having a first sloped portion extending from the initialpositive voltage to a terminal positive voltage having magnitude greaterthan or equal to zero volts; and generating a negative voltage phasehaving an initial negative voltage having a magnitude less than or equalto zero volts extending from the terminal positive voltage of thepositive voltage phase, the negative voltage phase having a secondsloped portion extending from the initial negative voltage to a terminalnegative voltage having a magnitude less than or equal to zero volts.

In accordance with a still further aspect of the present invention,there is provided a defibrillation waveform generator that includes: anarrhythmia detector adapted to be coupled to a heart, the arrhythmiadetector delivering a detection signal in response to detectingfibrillation in the heart; a charging circuit coupled to a capacitor,the charging circuit charging the capacitor to a given voltage; acontroller operably coupled to the arrhythmia detector to receive thedetection signal, the controller delivering a first control signal, asecond control signal, and a third control signal in response toreceiving the detection signal; a voltage-to-frequency convertor coupledto the controller to receive the first control signal, thevoltage-to-frequency convertor delivering a frequency signal having afrequency correlative to the first control signal; a pulse widthmodulator coupled to the controller to receive the second control signaland coupled to the voltage-to-frequency convertor to receive thefrequency signal, the pulse width modulator delivering a pulse widthmodulated signal having a frequency correlative to the frequency signaland having a duty cycle correlative to the second control signal; and aswitching circuit adapted to be coupled between the capacitor and theheart, the switching circuit being coupled to the controller to receivethe third control signal and to the pulse width modulator to receive thepulse width modulated signal, the switching circuit controllablydischarging the capacitor across the heart to deliver a defibrillationwaveform in response to the third control signal and the pulse widthmodulated signal.

In accordance with a yet further aspect of the present invention, thereis provided a defibrillation waveform generator that includes: anarrhythmia detector adapted to be coupled to a heart, the arrhythmiadetector delivering a detection signal in response to detectingfibrillation in the heart; a charging circuit coupled to a firstcapacitor and to a second capacitor, the charging circuit charging thefirst capacitor and the second capacitor to a respective given voltage;a controller operably coupled to the arrhythmia detector to receive thedetection signal, the controller delivering a first control signal, asecond control signal, and a third control signal in response toreceiving the detection signal; a voltage-to-frequency convertor coupledto the controller to receive the first control signal, thevoltage-to-frequency convertor delivering a frequency signal having afrequency correlative to the first control signal; a pulse widthmodulator coupled to the controller to receive the second control signaland coupled to the voltage-to-frequency converter to receive thefrequency signal, the pulse width modulator delivering a pulse widthmodulated signal having a frequency correlative to the frequency signaland having a duty cycle correlative to the second control signal; and aswitching circuit adapted to be coupled between the first and secondcapacitors and the heart, the switching circuit being coupled to thecontroller to receive the third control signal and to the pulse widthmodulator to receive the pulse width modulated signal, the switchingcircuit controllably discharging the first capacitor across the heart todeliver a positive phase defibrillation waveform in response to thethird control signal and the pulse width modulated signal, and theswitching circuit controllably discharging the second capacitor acrossthe heart to deliver a negative phase defibrillation waveform inresponse to the third control signal and the pulse width modulatedsignal.

In accordance with another aspect of the present invention, there isprovided a cardiac stimulator for treating fibrillations. The cardiacstimulator includes an implantable case containing: an atrial sensingcircuit adapted to deliver an atrial signal correlative to a conditionof an atrium of a heart; a ventricular sensing circuit adapted todeliver a ventricular signal correlative to a condition of a ventricleof the heart; an inductor-less pulse generator adapted to deliver pulsewidth modulated electrical stimulation to the ventricle; and a controlcircuit coupled to the ventricular sensing circuit to receive theventricular signal, the control circuit directing the pulse generator todeliver pulse width modulated electrical stimulation to the ventricle inresponse to classifying a ventricular tachyarrhythmia as a fibrillation.

In accordance with still another aspect of the present invention, thereis provided a cardiac stimulator that includes: means for determiningwhether a fibrillation exists in a ventricle; means for charging atleast one capacitor; and means for discharging the at least onecapacitor in a pulse width modulated manner to the ventricle to create adefibrillation waveform for treating the fibrillation.

In accordance with yet another aspect of the present invention, there isprovided a method of treating fibrillation that includes the acts of:(a) determining whether a fibrillation exists in a ventricle; (b)charging at least one capactitor; and (c) electrically stimulating theventricle with a waveform to treat the fibrillation by discharging theat least one capacitor in a pulse width modulated manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 illustrates a cardiac stimulator having two leads coupled to apatient's heart;

FIG. 2 illustrates a block diagram of one embodiment of a cardiacstimulator's circuitry in accordance with the present invention;

FIG. 3 illustrates a diagram of a heart rate spectrum that ispartitioned into various arrhythmia classes;

FIG. 4 illustrates a portion of a conventional monophasic defibrillationwaveform;

FIGS. 5-7 illustrate three positive voltage phase components of abiphasic waveform;

FIGS. 8-13 illustrate six negative voltage phase components of abiphasic waveform;

FIGS. 14-31 illustrate eighteen biphasic defibrillation waveformscreated from combining the three positive voltage phase componentsillustrated in FIGS. 5-7 with the six negative voltage phase componentsillustrated in FIGS. 8-13;

FIG. 32 illustrates a detailed version of the waveform illustrated inFIG. 14;

FIG. 33 illustrates a first embodiment of a circuit for generatingbiphasic defibrillation waveforms, such as the waveforms illustrated inFIGS. 14-31;

FIG. 34 illustrates a second embodiment of a circuit for generatingbiphasic defibrillation waveforms, such as the waveforms illustrated inFIGS. 14-31;

FIG. 35 illustrates a third embodiment of a circuit for generatingbiphasic defibrillation waveforms, such as the waveforms illustrated inFIGS. 14-31;

FIG. 36 illustrates a pulse train generated by the circuit illustratedin FIG. 34; and

FIG. 37 illustrates a waveform in the heart produced by the pulse trainillustrated in FIG. 36.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Turning now to the drawings, and referring initially to FIG. 1, oneembodiment of a dual-chamber cardiac stimulator is illustrated andgenerally designated by the reference numeral 10. As discussed below,the cardiac stimulator 100 advantageously includes a defibrillatorcircuit that produces one or more waveforms for treating a detectedfibrillation. The general structure and operation of the cardiacstimulator 10 will be discussed with reference to FIGS. 1-3. Then,various waveforms for treating fibrillation will be discussed withreference to FIGS. 4-32. Once these waveforms have been described,various exemplary methods and circuits for creating these waveforms willbe described with reference to FIGS. 33-37.

As shown in FIG. 1, the body of the cardiac stimulator 10 includes acase 12 and a header 14. The cardiac stimulator 10 may be implantable ornon-implantable. If implantable, the case 12 and the header 14 arehermetically sealed to prevent bodily fluids from damaging the internalcircuitry of the cardiac stimulator 10. Typically, the case 12 is madeof titanium, and the header 14 is made of polyethylene.

In the described embodiment, the cardiac stimulator 10 is a dual chambercardioverter/defibrillator defibrillator (ICD), although it should beunderstood that the teachings set forth herein may apply to other typesof cardiac stimulators, such as an implantable defibrillator or anexternal, stand-alone defibrillator for example. Because the cardiacstimulator 10 is a dual chamber ICD, it includes an atrial lead 16 and aventricular lead 18. Typically, the leads 16 and 18 are generallyflexible and include an electrically conductive core surrounded by aprotective sheath. For instance, the internal core may be a coiled wireof titanium, and the protective sheath may be a coating of polyurethaneor silicone.

Each lead 16 and 18 includes a respective tip 20 and 22 that is designedto be implanted or coupled to an interior surface of a chamber of theheart 24. As illustrated, the tip 20 of the atrial lead 16 is implantedin an inner wall of the right atrium 26 of the heart 24 for sensingand/or stimulating the right atrium 26. Similarly, the tip 22 of theventricular lead 18 is implanted in an inner wall of the right ventricle28 of the heart 24 for sensing and/or stimulating the right ventricle28.

The cardiac stimulator 10 uses electronic circuitry to perform itsfunctions, such as the circuitry illustrated in FIG. 2 and generallydesignated by the reference numeral 30. A microprocessor 32 providespacemaker control and computational facilities. Although it will beappreciated that other forms of circuitry, such as analog or discretedigital circuitry, can be used in place of microprocessor 32, amicroprocessor is typically advantageous due to its miniature size andits flexibility. Energy efficient microprocessors, which are designedspecifically for use in pacemakers, are particularly advantageous.

The microprocessor 32 has input/output ports connected in a conventionalmanner via bidirectional bus 34 to memory 36, an AV interval timer 38,and a pacing interval timer 40. In addition, the AV interval timer 38and pacing interval timer 40 each has an output connected to acorresponding input port of the microprocessor 32 by lines 42 and 44respectively. Memory 36 may include both ROM and RAM, and themicroprocessor 32 may also contain additional ROM and RAM. The pacemakeroperating routine is typically stored in ROM, while the RAM storesprogrammable parameters and variables in conjunction with the pacemakeroperation.

The AV and pacing interval timers 38 and 40 may be external to themicroprocessor 32, as illustrated, or internal thereto. The timers 38and 40 may be, for instance, suitable conventional up/down counters ofthe type that are initially loaded with a count value and count up to ordown from the value and output a roll-over bit upon completing theprogrammed count. The initial count value is loaded into the timers 38,40 on bus 34 and the respective roll-over bits are output to themicroprocessor 32 on lines 42 and 44.

The microprocessor 32 typically also has an input/output port connectedto a telemetry interface 46 by line 48. The pacemaker, when implanted,is thus able to receive pacing and rate control parameters from anexternal programmer 35 and to send data to an external receiver ifdesired. Many suitable telemetry systems are known to those skilled inthe art.

The microprocessor output ports are connected to inputs of an atrialstimulus pulse generator 50 and a ventricular stimulus pulse generator52 by control lines 54 and 56, respectively. The microprocessor 32transmits pulse parameter data, such as amplitude and width, as well asenable/disable and pulse initiation codes to the generators 50, 52 onthe respective control lines. The microprocessor 32 also has input portsconnected to outputs of an atrial sense amplifier 58 and a ventricularsense amplifier 60 by lines 62 and 64 respectively.

The atrial and ventricular sense amplifiers 58, 60 detect occurrences ofP-waves and R-waves respectively.

The input of the atrial sense amplifier 58 and the output of the atrialstimulus pulse generator 50 are connected to a first conductor 66 whichis inserted in a first conventional lead 68. Lead 68 is inserted into aheart 70 intravenously or in any other suitable manner. The lead 66 hasan electrically conductive pacing/sensing tip 72 at its distal end whichis electrically connected to the conductor 66. The pacing/sensing tip 72is typically lodged in the right atrium 74.

The input of the ventricular sense amplifier 60 and the output of theventricular stimulus pulse generator 52 are connected to a secondconductor 76. The second conductor 76 is inserted in a secondconventional lead 78 which is inserted intravenously or otherwise in theright ventricle 80 of the heart 70. The second lead 78 has anelectrically conductive pacing/sensing tip 82 at its distal end. Thepacing/sensing tip 82 is electrically connected to the conductor 76. Thepacing/sensing tip 82 is typically lodged on the wall of the rightventricle 80.

The conductors 66 and 76 conduct the stimulus pulses generated by theatrial and ventricular stimulus pulse generators 50, 52, respectively,to the pacing/sensing tips 72, 82. The pacing/sensing tips 72, 82 andcorresponding conductors 66, 76 also conduct sensed cardiac electricalsignals in the right atrium and right ventricle to the atrial andventricular sense amplifiers 58, 60.

To provide defibrillation capability in the cardiac stimulator 10, ahigh voltage defibrillator circuit 84 is provided which is controlled bythe microprocessor 32. The defibrillator circuit 84 is connected toheart tissue through two high voltage leads 86, 88 which communicatewith the heart through electrodes 90, 92. In the illustrated embodiment,epicardial patch electrodes are diagrammatically represented. However,other electrode configurations, including endocardial electrodes, mayalso be suitable. In fact, certain leads may be suitable for deliveringpacing pulses as well as defibrillation pulses, thus rendering the leads86 and 88 and the electrodes 90 and 92 superfluous. One example ofsuitable leads is disclosed in U.S. Pat. No. 5,476,502, the entirety ofwhich is hereby incorporated by reference.

The atrial and ventricular sense amplifiers 58, 60 communicate both withthe microprocessor and with a compressed signal A-to-D converter 94. Thecompressed signal A-to-D converter 94 communicates through the bus 34with memory 36 and the microprocessor 32, primarily, and on a line 96with the telemetry 46. Thus, the output of the converter 94 can bemanipulated by the microprocessor 32, or stored in memory 36 or directlycommunicated through the telemetry 46 to the programmer 35. The storedoutput of the convertor 94 may also be subsequently communicated frommemory 36 through the telemetry 46 to the programmer 35.

The microprocessor 32 may also base its control on other parameters,such as information received from other sensors. For example, anactivity sensor 98, such as an implanted accelerometer, may be used togather information relating to changing environmental or physiologicalconditions. Although the use of an accelerometer as the activity sensor98 may be advantageous, other types of sensors may also be used to gaugecertain types of physical activity or physical condition, such as“displacement” sensors, temperature sensors, oxygen sensors, pH sensors,and/or impedance sensors. Indeed, when the dual-chamber cardiacstimulator 10 is operating in rate-responsive mode, the stimulator 10typically adjusts the pacing rate in response to one or more detectedphysiological or environmental parameters correlated to a physiologicneed.

The operation of the cardiac stimulator 10 may be affected by heartrate. With reference now to FIG. 3, a heart rate spectrum may be storedin the circuitry 30 and partitioned into a multiplicity of regionsdefining contiguous, successive heart rate ranges. At the lower end ofthe illustrated heart rate spectrum is normal rhythm, which isdesignated SINUS. As the heart rate rises along the spectrum, thespectrum enters progressively higher rate ranges associated withventricular tachycardia or tachyarrhythmia, respectively labeled TACH-1,TACH-2, and TACH-3. Beyond the ventricular tachycardia ranges of thespectrum lies the range associated with ventricular fibrillation, whichis labeled FIB.

It will be observed that the spectrum may be partitioned such that therate ranges are representative of respective degrees of hemodynamictolerance of the patient to cardiac rates in those regions. Generallyspeaking, heart rates in the SINUS region are normal, whereas rates inthe FIB region cannot be tolerated. Furthermore, the ascending order ofthe three illustrated ventricular tachyarrhythmia regions TACH-1,TACH-2, and TACH-3 depicts well tolerated, moderately tolerated, andpoorly tolerated classes of tachycardia, respectively. Although threetachyarrhythmia classes are illustrated, the actual number of suchclasses may be greater or fewer depending on the judgment of thephysician regarding the management of arrhythmias and the prescriptionof therapy regimens for a particular patient. As will become clear fromthe discussion of therapy considerations below, the fibrillation rangeFIB is of particular concern for the purposes of this discussion.

When the cardiac stimulator 10 detects a heart rate in the fibrillationrange FIB, the defibrillator circuit 84 generates one or moredefibrillation waveforms that are delivered to the heart via theappropriate leads. The type, shape, and intensity of a defibrillationwaveform are determinative of the efficacy of the treatment of thefibrillated heart. Regarding the type of waveform, the advantages of abiphasic waveform over a monophasic waveform have been discussedpreviously. To illustrate certain differences between these types ofwaveforms, a monophasic waveform 100 is illustrated in FIG. 4.

As can be seen, a typical monophasic waveform rises from zero volts to acertain maximum voltage V_(max) and exhibits a certain oscillatorycharacteristic about an average voltage V_(avg) before returning to zerovolts. As compared to the biphasic waveforms illustrated in thesubsequent figures, the maximum voltage V_(max) necessary for themonophasic waveform 100 to overcome the defibrillation threshold tendsto be greater, possibly by fifty percent or more, than the maximumvoltage of the various biphasic waveforms. As discussed previously, thisproblem is primarily due to the generation of a polarized boundarybetween the electrodes and the heart caused by the large net voltagedelivery of the monophasic waveform 100.

However, the primary advantage of the monophasic waveform 100 is that itmay be generated by capacitive discharge circuitry that is small enoughto be used within an implantable cardiac stimulator. At present, thecircuitry for producing suitable biphasic defibrillation waveformstypically utilizes a massive circuit containing a very large inductor tocreate the appropriately shaped waveforms. Such circuitry is notfeasible for use in an implantable device.

Furthermore, the types of biphasic waveforms previously considered foruse in treating fibrillation are somewhat limited and are notprogrammably selectable for treating various types of fibrillation. Inthe discussion below, various biphasic waveform components aredescribed, as well as the various combinations of resulting biphasicwaveforms. The nature of each of these waveforms will be discussed,along with exemplary situations in which certain waveforms may proveefficacious. Then, three exemplary circuits will be described forproducing a variety of biphasic waveforms, including the illustratedbiphasic waveforms.

FIGS. 5-7 illustrate three different positive voltage components of abiphasic waveform that may be used for defibrillation. The positivevoltage component 102 illustrated in FIG. 5 includes a linearly slopedportion 104. The linearly sloped portion 104 begins at some valuegreater than or equal to zero volts and rises in a linear fashion to amaximum voltage dependent upon the given slope and the duration of thepositive phase of the waveform. It is believed that a positive voltagewhich ramps up to its maximum voltage may lower the defibrillationthreshold, thus reducing the maximum voltage necessary to defibrillatethe heart. Thus, the shape of the waveform component 102 may exhibitadvantages over the monophasic waveform 100.

The waveform component 106 illustrated in FIG. 6 also ramps up to amaximum positive voltage. However, unlike the waveform component 102,the waveform component 106 includes a portion 108 which ramps upwardlywith a variably decreasing slope. The portion 108 begins its rise at avoltage greater than or equal to zero volts and increases to its maximumvoltage at the end of the positive phase. Given the same initialvoltage, it should be appreciated that the waveform component 106carries slightly more power than the waveform component 102.

Another variant of a positive voltage component of a biphasic waveformis illutrated in FIG. 7 as a waveform component 110. The waveformcomponent 110 is illustrated as including a sloped portion 112 thatdecreases at an exponential rate from an initial maximum voltage to afinal voltage that is greater than or equal to zero volts. It should beappreciated that the sloped portion 112 of the waveform component 110alternatively may be positively sloped in an exponentially increasingfashion from an initial voltage that is greater than or equal to zerovolts to some maximum voltage at the end of the phase.

It is currently believed that most patients will respond adequately toone or more of the waveform components 102, 106, and 110. Thus, incombination with the negative voltage phases described below withreference to FIGS. 8-13, a physician may program one or moredefibrillation waveforms into the cardiac stimulator 10 to provide apatient with the most efficacious treatment.

Various negative voltage waveform components are illustrated in FIGS.8-13. As described in more detail below, these six negative voltagecomponents may be combined with the three positive voltage componentsdescribed above to form the eighteen waveforms illustrated in FIGS.14-31. FIG. 8 illustrates a negative voltage waveform component 114. Thecomponent 114 includes a positively sloping portion 116 that increasesat a linear rate from a maximum negative voltage to a smaller negativevoltage that is less than or equal to zero volts. FIG. 9 illustrates anegative voltage waveform component 118 which includes a negativelysloping portion 120 that linearly decreases from an initial voltage lessthan or equal to zero volts to some maximum negative voltage. FIG. 10illustrates a waveform 122 that includes a portion 124 that slopesnegatively in an exponential manner from an initial voltage less than orequal to zero volts to a maximum negative voltage. FIG. 11 illustrates awaveform 126 that includes a portion 128 that slopes positively in anexponential manner from an initial maximum negative voltage to a smallernegative voltage less than or equal to zero volts. FIG. 12 illustrates awaveform 130 that includes a portion 132 that ramps upwardly with avariably decreasing slope from a maximum negative voltage to a smallernegative voltage less than or equal to zero volts. Finally, FIG. 13illustrates a waveform 134 that includes a sloped portion 136 similar tothe portion 132 where the final negative voltage is less than zerovolts.

Referring additionally now to FIGS. 14-31, it can be seen that thewaveforms 140-150 illustrated in FIGS. 14-19 are comprised of thepositive voltage waveform component 102 in combination with the negativevoltage waveform components 114, 118, 122, 126, 130, and 134,respectively. Similarly, the waveforms 152-162 illustrated in FIGS.20-25 are composed of the positive voltage waveform component 106 incombination with the negative voltage waveform components 114, 118, 122,126, 130, and 134, respectively, and the waveforms 164-174 illustratedin FIGS. 26-31 are composed of the positive voltage waveform component110 in combination with the negative voltage waveform components 114,118, 122, 126, 130, and 134, respectively.

Each of the waveforms 140-174 illustrated in the respective FIGS. 14-31have certain advantages that may make a particular waveform advantageousfor treating fibrillation. For example, the waveforms 140-162 have apositively sloped first phase. This slow ramp up of the positive voltagein the first phase tends to minimize polarization of theelectrode-tissue interface and results in more efficient energytransfer. Using conventional techniques, the first phase of thewaveforms 152-162 would be easier to generate than the first phase ofthe waveforms 140-150 using analog circuitry, primarily because thelatter requires the use of an inductor. Similarly, the second phase ofthe waveforms 140, 142, 144, 152, 158, 156, 164, and 166 is moredifficult to generate using conventional analog methods than the otherwaveforms because such generation requires the use of an inductor in thecircuit. Conversely, using conventional techniques, the generation ofthe first phase of the waveforms 164-174 could be generated using acapacitive discharge. However, as will be explained in detail below, thepresently disclosed circuits illustrated in FIGS. 34 and 35 can generateeach of the waveforms 140-174 through the use of a capacitive dischargewithout the use of an inductor.

In regard to additional advantages, it should also be noted that thewaveforms 140, 146, 148, 150, 152, 158, 160, and 162 have a large,abrupt gradient between the positive peak of the first phase and thenegative peak of the second phase. This gradient typically promotes moreeffective defibrillation. Indeed, each of these waveforms has anascending slope in the first phase followed by the large gradient. Asthe slope rises, by virtue of its capacitance (dV/dt), the tissue isprepared for the abrupt change in the amplitude and direction of thevoltage which occurs between the two phases. Thus, these waveformscombine two advantageous characteristics leading to low defibrillationthresholds, i.e., a slow ramp up of the positive voltage to minimizepolarization to reduce the threshold combined with a large voltagegradient to overcome the threshold.

It should be further understood that the waveforms 140-174 are eachconsidered to be generic in the sense that the amplitudes, widths, timeconstants, and delays between phases may vary somewhat to provideefficacious defibrillation so long as the basic shape of the respectivewaveform is maintained. To demonstrate this, a detailed discussion ofthe waveform 140 is set forth below with reference to FIG. 32. However,it should be understood that similar statements are applicable to eachof the waveforms 140-174.

As illustrated in FIG. 32, the waveform 140 has an initial amplitude180, typically between 0 volts and 50 volts. The amplitude slopesupwardly as a ramp 182 until it reaches a maximum amplitude 184,typically between about 200 volts and 400 volts, that is greater thanthe initial amplitude 180. Advantageously, the slope of the ramp 182 isgreater than zero but need not be precisely linear, such as less than 70volts per millisecond. At the beginning of the second phase, thewaveform 140 transitions from the maximum positive amplitude 184 to themaximum negative amplitude 186, typically between about −200 volts and−400 volts. The amplitude of the waveform 140 then slopes upwardly inthe form of a ramp 188 to a final negative amplitude 190, typicallybetween 0 volts and −50 volts. Again, the slope of the ramp 188 isadvantageously greater than zero but need not be precisely linear, suchas less than 70 volts per millisecond.

The waveform 140 may also exhibit an interphase delay 192 that isgreater than or equal to zero. Advantageously, the interphase delay 192is as close to zero as possible, but it may have some small positivevalue due to the manner in which the circuit that generates the waveform140 operates. Finally, the widths 194 and 196 of the positive andnegative phases are greater than zero. Indeed, the amplitudes 180, 184,186, and 190 and the widths 194 and 196 are typically selected toprovide efficacious defibrillation while minimizing power consumption.

The waveforms 140 and 150 are of particular interest, because thesewaveforms along with waveforms 146, 148, 152, 158, 160, and 162, have alarge gradient between the positive peak of the first phase and thenegative peak of the second phase. It is currently believed that thislarge gradient, along with an appropriate selection of slope, promotesmore effective defibrillation than other biphasic waveforms. Indeed, theresults of certain tests support this belief.

During testing on eight canine subjects, it has been determined that thewaveform 150 provides more efficacious defibrillation than the waveform174. Before discussing details of this test, it should be noticed thatthe waveform 150 differs from the waveform 174 primarily in the positivephase. Indeed, as illustrated, the negative phase of the waveforms 150and 174 are identical. However, it should be noted that the positivephase of the waveform 150 begins at a nominal positive value and slopespositively to a maximum positive value before transitioning to a maximumnegative value in the negative phase. In contrast, the waveform 174includes a positive phase which begins at its maximum positive value andslopes negatively to a lower positive value before transitioning to themaximum negative value in the negative phase.

The purpose of this test was to examine the effects of rising edgewaveforms on defibrillation thresholds, commonly referred to as DFT. Inthis test, the defibrillation efficacy of a test waveform generallycorresponding to the waveform 150 was compared to a capacitive dischargereference waveform generally corresponding to the waveform 174. Thereference waveform was a biphasic, truncated, capacitive dischargewaveform with a first phase having a duration of 6.5 milliseconds and asecond phase having a duration of 3.5 milliseconds, with a delay betweenphases of 0.08 milliseconds. The capacitance of both phases was fixed at125 microfarads. In as much as the capacitance and pulse widths werefixed, the load impedance and the peak voltage of the first phasegenerally determine the total energy delivered in both phases.

The test waveform was a biphasic waveform having a positive first phaseand a negative second phase with a delay between phases of 0.1milliseconds. Although the rising edge waveform in the first phasediffers from the capacitive discharge waveform in the second phase, thepeak voltage of the negative phase was correlated as closely as possibleto the peak voltage of the first phase. However, due to energy deliveryconsiderations, the peak voltage of the negative phase was varied by asmuch as ±20 percent from the peak voltage of the positive phase. Thenegative phase of the test waveform remained identical to the negativephase of the reference waveform throughout the test. However, theduration and slope of the positive phase was varied throughout the testas set forth in the table below.

TABLE 1 BIPHASIC WAVEFORM DESIGN CONDITION SLOPE OF PULSE WIDTH OFNUMBER FIRST PHASE POSITIVE/NEGATIVE PHASE 1 40 V / ms. 3 ms. / 3.5 ms.2 80 V / ms. 3 ms. / 3.5 ms. 3 40 V / ms. 4 ms. / 3.5 ms. 4 80 V / ms. 4ms. / 3.5 ms. 5 40 V / ms. 5 ms. / 3.5 ms. 6 80 V / ms. 5 ms. / 3.5 ms.

In the test, the reference waveform was used to define a referenceenergy corresponding to a defibrillation threshold. Once this referenceenergy was defined, a search algorithm applied test waveforms randomlyfrom about 50 percent to about 200 percent of the reference energydefined by the reference waveform. When the data from the test waveformsand the reference waveform was analyzed and compared, the data clearlyshowed a reduction of the defibrillation threshold associated with thetest waveform. Specifically, the data demonstrated that the testwaveform was particularly efficacious where the peak positive voltagewas greater than 300 volts with the slope less than 70 volts permillisecond. Indeed, the data demonstrated a trend for lower slop valuecoupled with higher peak voltages to be more efficacious fordefibrillation. Overall, by prosper selection of the peak voltage in thepositive phase and slope value, it was found that the defibrillationthreshold can be reduced significantly, as the data showed reductions of10 percent to 50 percent as compared to the reference waveform.

FIG. 33 illustrates a circuit 200 that is capable of generating thewaveforms 140-174. The circuit 200 includes three subcircuits forproducing the desired waveforms. The first subcircuit is a curve shapingcircuit 202, the second subcircuit is a charging circuit 204, and thethird subcircuit is a biphasic switching circuit 206

In regard to the curve shaping circuit 202, the values of the capacitors208 and 210, the inductor 212, and the resistor 214 are chosen toproduce the desired discharge curve which corresponds to the slopedportions 104, 108, 112, 116, 120, 124, 128, 132, and 136 of therespective waveforms 102, 106, 110, 114, 118, 122, 126, 130, and 134.For example, the values of the inductor 212 and the capacitor 208 can bemodified to produce the waveforms 148 and 152. The resistor 214essentially operates as a “dummy” load which is used to modify thedischarge characteristics of the capacitor 208. Finally, the filtercircuit 216, which is typically a capacitive circuit, may be used tolinearize the discharge output to form ramped sections as illustrated inthe waveforms 102, 114, and 118, for example. Even more particularly, itshould be understood that the resistance 218 provided by the heart alongwith the values of the capacitor 208 and the inductor 212 generallydetermine the initial shape of the waveform. While the resistor 214 maybe used to change the waveform shape, it should also be noted thatchanges in the value of the capacitor 208 or the value of the inductor212, whether by using variable elements, parallel elements, or elementsof different values, may also affect the waveform shape.

To charge the elements in the curve shaping circuit 202, the switch 218is initially placed in an open state so that the power supply 220 cancharge the capacitor 208. Similarly, the charging circuit 204 closes theswitch 222 so that the power supply 224 can charge the capacitor 210. Inthis embodiment the state of the switches 218 and 222 are controlled bypulse generators 226 and 228, respectively. However, it should beunderstood that the switches 218 and 222 alternatively may be controlledby a variety of other suitable methods, such as an appropriate logiccircuit, state machine, or microprocessor.

Once the capacitors 208 and 210 have been charged, the switches 218 and222 are simultaneously opened to allow the respective capacitors 208 and210 to discharge. To produce the positive voltage of the first phase ofthe biphasic waveform, the switches 230 and 232 of the switching circuit206 are closed, and the switches 234 and 236 are opened. As before, thestate of the switches in the charging circuit 206 are controlled bypulse generators 238 and 240, but other methods of control may besuitable.

With the switches 230-236 in this configuration, current flows throughthe switch 230, through the heart 218, and through the switch 232 tocomplete the circuit. This current is designated as the first phasecurrent which produces the positive voltage waveform in the heart 218.To complete phase one and begin phase two, the switches 230 and 232 areopened, and the switches 234 and 236 are closed. With the switches inthis configuration, current flows through the switch 236, through theheart 218, and through the switch 234 to complete the circuit. Thisphase two current produces the negative voltage waveform in the heart218.

Although the circuit 200 is capable of producing any of the biphasicwaveforms 140-174 illustrated in FIGS. 14-31, it does suffer fromcertain disadvantages. First, the circuit 200 includes at least oneinductor 212. Because it is difficult, if not commercially impossible,to fabricate an inductor of sufficient value to produce the necessarywaveform while still being small enough to fit within an implantablecardiac stimulator, the circuit 200 may not be suitable for animplantable device. Secondly, without the use of multiple elements208-214 and/or variable elements 208-214, the circuit 200 can onlyproduce a waveform having a specified shape. Even if multiple orvariable elements were used, these elements would occupy even morespace, making the circuit 200 even less suitable for an implantabledevice.

The circuits 250 and 300 illustrated in FIGS. 34 and 35 have beendesigned to address these problems. The circuits 250 and 300 use acombination of pulse width modulation (PWM) and frequency modulation(FM) to generate a plurality of waveforms suitable for defibrillatingventricular or atrial tissue. Through the proper choice of these PWM andFM parameters, and through the proper selection of capacitor chargingparameters, the circuits 250 and 300 can generate a number of suitablemonophasic or biphasic waveforms such as the waveforms 140-174.Particularly, the circuits 250 and 300 can produce a rising edgewaveform that begins at some value greater than or equal to zero voltsand increases linearly to some maximum voltage dependent upon the slopeand duration of the first phase. The circuits 250 and 300 can alsoproduce, in a similar manner, a second phase waveform where the initialvoltage is dependent upon the final voltage of the capacitor used togenerate the first phase (circuit 250 of FIG. 34) or upon the chargevoltage of a second capacitor (circuit 300 of FIG. 35).

In regard to the circuit 250 illustrated in FIG. 34, an arrhythmiadetector 252 is coupled to the heart which is symbolized by a resistance254. If the arrhythmia detector 252 detects a treatable arrhythmia inthe heart 254, it issues a command to the system controller 256 via aline 257. The system controller 256, typically the microprocessor 32,signals a charging circuit 258 via a line 260 to initiate the chargingof a capacitor 262. The charging circuit 258 then charges the capacitor262 to a preprogrammed voltage. When the capacitor 262 reaches thedesired voltage level, the system controller 256 closes a switch 264 andmodulates the opening and closing of a switch 266 to control delivery ofa defibrillation signal to the heart. The controller 256 controls themodulation of the switch 266 by delivering a signal on line 270 to avoltage-to-frequency converter 268. The magnitude of the voltagedelivered to the voltage-to-frequency converter 268 by the controller256 controls the frequency of the signal delivered by thevoltage-to-frequency converter 268 on line 272. The frequency isadvantageously in a range between about 5 kilohertz and about 25kilohertz. A pulse width modulator 274 receives the frequency signalfrom the line 272, and it also receives a control signal from thecontroller 256 via line 276. The pulse width modulator 274 delivers apulse width modulated signal to the switch 266. The frequency of thepulse width modulated signal is controlled by the frequency of thesignal on line 272, while the duty cycle of the pulse width modulatedsignal is controlled by the control signal delivered on line 276. Eachtime the pulse width modulated signal is high, the switch 266 closes sothat current passes from the capacitor 262 through the switch 264,through the heart 254, and through the switch 266 to complete thecircuit. Thus, the waveforms are produced by essentially modulating thecurrent discharged by the capacitor 262.

An example of actual waveform characteristics is illustrated in FIG. 36.Here, it can be seen that the first pulse 278 of the waveform has avoltage magnitude equal to the fully charged voltage of the capacitor262, with the pulse width determined by the duty cycle of the pulsewidth modulated signal delivered to the switch 266. The next pulse 280has a lower magnitude because the capacitor 262 was partially dischargedin order to form the first pulse 278. As before, the width of the secondpulse 280 is determined by the duty cycle of the pulse width modulatedsignal delivered to the switch 266. Furthermore, the time delay betweenthe first pulse 278 and the second pulse 280 is determined by thefrequency of the pulse width modulated signal delivered to the switch266. As successive pulses 282 are delivered, the magnitude of each pulsetends to diminish as the capacitor 262 continues to discharge. The dutycycle of the pulse width modulated signal continues to determine thewidth of each successive pulse 282, while the frequency of the pulsewidth modulated signal continues to determine the time at which eachpulse edge occurs.

Each of these pulses is applied to the heart via the path describedabove. Notwithstanding the fact that the heart 254 has been illustratedby a resistor, it is believed that myocardial tissue and individualcells essentially act as low pass filters in the sense that they rejectfrequencies higher than approximately two kilohertz to about fivekilohertz. In other words, the heart 254 does not respond to each pulseso long as the pulses are being applied to it at a suitably highfrequency. Instead, the heart tends to filter the pulses and integratethe power in the pulse train. Thus, the pulse train illustrated in FIG.36 will generate a voltage through the heart similar to the waveform 284illustrated in FIG. 37. However, it should be understood that by varyingthe timing and width of the pulse train virtually any waveform can becreated by the circuit 250.

Phase two of a biphasic waveform is produced by the circuit 250 in muchthe same way as the phase one component of the waveform. Specifically,upon completion of the phase one component, the controller 256 opens theswitches 264 and 266, closes the switch 286, and modulates the openingand closing of the switch 288. Similar to the operation described above,the switch 286 remains on at all times during phase two, while theswitch 288 receives a pulse width modulated signal from the pulse widthmodulator 274. The controller 256 controls the pulse width modulatedsignal in the same manner as described previously in order to producethe desired shape of the phase two component of the waveform. Hence,current flows from the capacitor 262 through the switch 286, through theheart 254, and through the switch 288 to complete the circuit.

The operation of the circuit 300 illustrated in FIG. 35 is similar tothe operation of the circuit 250 described above, with the primaryexception of the use of a dedicated capacitor to generate eachrespective phase of the biphasic waveform. As illustrated in FIG. 35, anarrhythmia detector 302 is coupled to the heart which is symbolized by aresistance 304. If the arrhythmia detector 302 detects a treatablearrhythmia in the heart 304, it issues a command to the systemcontroller 306 via a line 307. The system controller 306 signals acharging circuit 308 via a line 310 to initiate the charging of thecapacitors 312 and 314. The charging circuit 308 then charges thecapacitors 312 and 314 to a respective preprogrammed voltage. When thecapacitors 312 and 314 reach the desired voltage levels, the systemcontroller 306 closes a switch 316 and modulates the opening and closingof a switch 318. The controller 306 controls the modulation of theswitch 318 by delivering a signal to a voltage-to-frequency converter320 on line 322. The magnitude of the voltage delivered to thevoltage-to-frequency converter 320 by the controller 306 controls thefrequency of the signal delivered by the voltage-to-frequency converter320 on line 324. The frequency is advantageously in a range betweenabout 5 kilohertz and about 25 kilohertz. A pulse width modulator 326receives the frequency signal from the line 324, and it also receives acontrol signal from the controller 306 via line 328. The pulse widthmodulator 326 delivers a pulse width modulated signal to the switch 318.The frequency of the pulse width modulated signal is controlled by thefrequency of the signal on line 324, while the duty cycle of the pulsewidth modulated signal is controlled by the control signal delivered online 328. Each time the pulse width modulated signal is high, the switch318 closes so that current passes from the capacitor 312 through theswitch 316, through the heart 304, and through the switch 318 tocomplete the circuit.

The negative phase of a biphasic waveform is produced by the circuit 300in much the same way as the positive phase component of the waveform.Specifically, upon completion of the phase one component, the controller306 opens the switches 316 and 318, closes the switch 330, and modulatesthe opening and closing of the switch 332. Similar to the operationdescribed above, the switch 330 remains closed at all times during phasetwo, while the switch 332 receives a pulse width modulated signal fromthe pulse width modulator 326. The controller 306 controls the pulsewidth modulated signal in the same manner as described previously inorder to produce the desired shape of the phase two component of thewaveform. However, unlike the circuit 250, in the circuit 300 currentflows from the second capacitor 314 through the switch 330, through theheart 304, and through the switch 332 to complete the circuit. Thus, thenegative phase is not reliant on the charge left in the first capacitor312 at the end of the positive phase.

Specific embodiments of the invention have been shown by way of examplein the drawings and have been described in detail herein. However, theinvention may be susceptible to various modifications and alternativeforms, and it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A defibrillator comprising: a biphasic voltagewaveform generator circuit, the circuit generating a waveform thatincludes: a positive voltage phase beginning at about zero volts andhaving an initial positive voltage magnitude greater than zero volts,the positive voltage phase having a first positively sloped portionextending from the initial positive voltage magnitude to a maximumpositive voltage magnitude greater than the initial positive voltagemagnitude; and a negative voltage phase having an initial maximumnegative voltage magnitude less than zero volts extending from themaximum positive voltage magnitude of the positive voltage phase, thenegative voltage phase having a second positively sloped portionextending from the initial maximum negative voltage magnitude to aterminal negative voltage magnitude less than the initial maximumnegative voltage magnitude.
 2. The defibrillator, as set forth in claim1, wherein the initial positive voltage magnitude is in a range fromabout 0 volts to about 50 volts.
 3. The defibrillator, as set forth inclaim 1, wherein the maximum positive voltage magnitude is in a rangefrom about 200 volts to about 400 volts.
 4. The defibrillator, as setforth in claim 1, wherein the initial maximum negative voltage magnitudeis in a range from about −200 volts to about −400 volts.
 5. Thedefibrillator, as set forth in claim 1, wherein the terminal negativevoltage magnitude is in a range from about −50 volts to about 0 volts.6. The defibrillator, as set forth in claim 1, wherein the firstpositively sloped portion comprises a substantially linear slope.
 7. Thedefibrillator, as set forth in claim 1, wherein the second positivelysloped portion comprises a substantially linear slope.
 8. Thedefibrillator, as set forth in claim 1, wherein the first positivelysloped portion comprises a continuously decreasing slope.
 9. Thedefibrillator, as set forth in claim 8, wherein the initial positivevoltage magnitude is in a range from about 0 volts to about 400 volts.10. The defibrillator, as set forth in claim 8, wherein the terminalpositive voltage magnitude is in a range from about 0 volts to about 400volts.
 11. The defibrillator, as set forth in claim 8, wherein theinitial negative voltage magnitude is in a range from about 0 volts toabout −400 volts.
 12. A defibrillator comprising: a biphasic voltagewaveform generator circuit, the circuit generating a waveform thatincludes: a positive voltage phase having an initial positive voltagehaving a magnitude greater than or equal to zero volts and having afirst sloped portion extending from the initial positive voltage to aterminal positive voltage having magnitude greater than or equal to zerovolts, the positive phase waveform shape independently selectable from afirst set of waveform shapes; and a negative voltage phase having aninitial negative voltage having a magnitude less than or equal to zerovolts extending from the terminal positive voltage of the positivevoltage phase, the negative voltage phase having a second sloped portionextending from the initial negative voltage to a terminal negativevoltage having a magnitude less than or equal to zero volts, thenegative waveform shape independently selectable from a second set ofwaveform shapes.
 13. The defibrillator, as set forth in claim 8, whereinthe first sloped portion comprises a positive slope.
 14. Thedefibrillator, as set forth in claim 13, wherein the first slopedportion comprises a substantially linear slope.
 15. The defibrillator,as set forth in claim 8, wherein the second sloped portion comprises apositive slope.
 16. The defibrillator, as set forth in claim 15, whereinthe second sloped portion comprises a substantially linear slope. 17.The defibrillator, as set forth in claim 8, wherein the waveformincludes an interphase delay between the positive voltage phase and thenegative voltage phase.
 18. The defibrillator, as set forth in claim 17,wherein the first sloped portion comprises a substantially linear slope.19. The defibrillator, as set forth in claim 17, wherein the firstsloped portion comprises a continuously increasing slope.
 20. Thedefibrillator, as set forth in claim 17, wherein the first slopedportion comprises a continuously decreasing slope.
 21. Thedefibrillator, as set forth in claim 12, wherein the first slopedportion comprises a negative slope.
 22. The defibrillator, as set forthin claim 21, wherein the first sloped portion comprises a substantiallylinear slope.
 23. The defibrillator, as set forth in claim 21, whereinthe first sloped portion comprises a continuously increasing slope. 24.The defibrillator, as set forth in claim 21, wherein the first slopedportion comprises a continuously decreasing slope.
 25. Thedefibrillator, as set forth in claim 12, wherein the second slopedportion comprises a positive slope.
 26. The defibrillator, as set forthin claim 25, wherein the second sloped portion comprises a substantiallylinear slope.
 27. The defibrillator, as set forth in claim 25, whereinthe second positively sloped portion comprises a continuously increasingslope.
 28. The defibrillator, as set forth in claim 25, wherein thesecond positively sloped portion comprises a continuously decreasingslope.
 29. The defibrillator, as set forth in claim 12, wherein thesecond sloped portion comprises a negative slope.
 30. The defibrillator,as set forth in claim 29, wherein the second sloped portion comprises asubstantially linear slope.
 31. The defibrillator, as set forth in claim29, wherein the second positively sloped portion comprises acontinuously increasing slope.
 32. The defibrillator, as set forth inclaim 29, wherein the second positively sloped portion comprises acontinuously decreasing slope.
 33. The defibrillator, as set forth inclaim 12, wherein the first sloped portion comprises a negative slope.34. The defibrillator, as set forth in claim 12, wherein the firstsloped portion comprises a continuously increasing negative slope. 35.The defibrillator, as set forth in claim 12, wherein the second slopedportion comprises a negative slope.
 36. The defibrillator, as set forthin claim 35, wherein the second sloped portion comprises a substantiallylinear negative slope.
 37. The defibrillator, as set forth in claim 35,wherein the second sloped portion comprises a continuously increasingnegative slope.
 38. The defibrillator, as set forth in claim 12, whereinthe waveform includes an interphase delay between the positive voltagephase and the negative voltage phase.